Cycles hair particles disappear after I comb the hair?
Particles will not render in particle edit mode, due to a limitation:


  This is intentional, it's a limitation in the particle system code that both Blender Internal and Cycles suffer from. Not considered a bug at the moment.


So make sure you are not in particle edit mode when rendering.



In case you are wondering what the suffering is about, blender's particle code is known to be inefficient and filled with issues. (I can't seem to find any sources for this, but I know I read it somewhere..)

A "looking into" is being talked about as part of the Gooseberry project, and a rewrite (with nodes support!) is also being worked on.